This invention is directed to a method of recovering the thermal energy of a coal formation by burning the coal in situ, generating steam with the resulting heat, and recovering the steam.
In U.S. Pat. No. 3,628,929 to Eugene D. Glass and Vaughan W. Rhoades, there is described a method for producing flammable gas and coal tar liquids by the in situ combustion of a coal seam. The process consists of completing wells in the coal seam, creating a horizontal fracture in the coal seam to establish communication in the wells, igniting the coal seam about the injection well, injecting a combustion-supporting gas, and recovering volatile liquids at the production well. A preferred method for controlling the temperature of the flame front and adjusting the calorific value of the produced gas is by the simultaneous injection of water with the combustion-supporting gas. In U.S. Pat. No. 3,775,073 to Vaughan W. Rhoades, there is described another process for the in situ combustion of a coal seam and the recovery of flammable gases and coal tar liquids therefrom. Two or more wells are completed in the coal seam and a horizontal fracture is created therein to establish communication between the wells. A first combustion-supporting gas is ignited within the horizontal fracture so as to form a horizontal and vertical fractured network between the wells. A second combustion-supporting gas is injected to propagate the combustion front through the coal about the injection well and the volatile liquids and combustible gases are recovered at the production well. There is disclosed a preferred method for controlling the temperature of the flame front and adjusting the calorific value of the produced gas by simultaneously injecting water with the combustion-supporting gas and obtaining a water-gas shift reaction at the site of the combustion front.
In U.S. Pat. No. 3,520,363 to Charles L. Bauer, there is described a process for the recovery of hydrocarbons from subterranean hydrocarbon-bearing formations by conducting an in situ combustion operation prior to water injection. In situ combustion is caused to occur in random pockets in a subterranean formation followed by water injection thereinto to scavenge the generated heat as part of a water drive and thereby produce additional hydrocarbons contained therein.
In U.S. Pat. No. 3,285,335 to Carl E. Reistle, Jr., there is described a method of in situ pyrolysis or recovery of organic carbonaceous materials from subterranean deposits of oil shale. A number of substantially horizontal shafts are drilled through an oil shale formation such that at least a pair of the shafts are in different horizontal planes and generally oriented in the same direction. Fracturing operations are conducted from at least one of the shafts so that the plane of the resulting fracture is vertically disposed so as to open communication between at least a pair of horizontal shafts. A thermal fluid having a temperature sufficient to pyrolyze the oil shale is introduced into the formation along the parted fractured plane whereby the sensible heat of the fluid is transferred to the surrounding oil shale formation and pyrolysis and vaporization of the kerogen content of the oil shale occur. The products of pyrolysis and vaporization are entrained in the thermal fluid and are recovered therewith at the earth's surface. In accordance with another aspect, the preferred fracture orientation in the oil shale formation is determined and the work shaft is drilled into the oil shale formation and from the work shaft a plurality of substantially horizontal shafts are drilled into the formation in order to penetrate the formation at angles not less than 30.degree. and as near as 90.degree. as possible relative to the preferred fracture orientation of the oil shale. The formation is then fractured from at least one of the horizontal shafts to open communication with at least one other of the horizontal shafts and thermal fluid is circulated to pyrolyze the oil shale and conduct the products of pyrolysis to the earth's surface.
In U.S. Pat. No. 3,835,928 to Malcolm K. Strubhar and Edwin E. Glenn, Jr., there is described a method for forming from a deviated well a plurality of vertically disposed fractures spaced a substantial distance apart in a subterranean formation having a known preferred fracture orientation.
In U.S. Pat. No. 3,863,709 to John L. Fitch, there is described a method and system for recovering geothermal energy from a subterranean geothermal formation having a preferred vertical fracture orientation wherein at least two deviated wells are provided which extend into the geothermal formation in a direction transversely of the preferred fracture orientation and a plurality of vertical fractures are hydraulically formed to intersect the deviated wells. A fluid is injected via one well into the fractures to absorb heat from the geothermal formation and the heated fluid is recovered from the formation via another well.
In a copending application, U.S. Ser. No. 660,079, filed Feb. 23, 1976, THERMAL ENERGY PRODUCTION BY IN SITU COMBUSTION OF COAL, by John L. Fitch and Malcolm K. Strubhar, there is disclosed a method of recovering thermal energy from a coal formation having a preferred vertical fracture orientation. An injection well and a production well are provided to extend into the coal formation and a vertical fracture is formed by hydraulic fracturing techniques and propagated into the coal formation to communicate with both wells. The vertical fracture is propped in the lower portion only. A combustion-supporting gas is injected into the propped portion of the fracture and the coal is ignited. Injection of the combustion-supporting gas is continued to propagate a combustion zone along the propped portion of the fracture and hot product gases generated at the combustion zone are produced to recover the heat or thermal energy of the coal. Water may also be injected into the fracture to transport the heat resulting from the combustion of the coal to the production well for recovery.